(1) Field of the Invention
The invention relates to anechoic composites as a coating or as a component of a structural element for use on undersea platforms.
(2) Description of the Prior Art
Presently, noise control technology for undersea vehicles includes external coatings in which the coatings absorb probing undersea sound waves produced by sonar transducers and thereby echoes of the undersea sound waves are minimized to prevent active detection of the undersea vehicles.
In Rauh (U.S. Pat. No. 3,698,993), a foamed closed cell sheet elastomeric material with particulate material distributed there through is disclosed. The particulate material is composed of high density particles of variegated sizes and shapes. The high density particles preferably have a specific gravity and are extruded. The particles are of irregular heterogeneous shape as distinguished from regular geometric shapes or patterns.
In Fischer et al. (U.S. Pat. No. 5,420,825), a composite for use on submarines and surface craft for controlling self-generated noise is disclosed. The composite includes two layers of PVF2 transducers separated by a layer of phase shifting or absorbing material. The inner transducer senses noise from the ship and subtracts this from the signal from the outer transducer representing noise plus the desired signal. In a second mode, the sensed noise is regenerated through the outer transducer 180 degrees out-of-phase to cancel the noise and allow more accurate detection.
In Cushman et al. (U.S. Pat. No. 5,400,296), an acoustic attenuation and vibration damping material is disclosed. Embedded within the material are high and/or low characteristic acoustic impedance particles in which the particles are mismatched to allow some portion of the impinging acoustic or vibratory energy to be reflected.
In Sevik (U.S. Pat. No. 5,444,668), an anechoic and decoupling coating for use on an underwater structure is disclosed. The coating is an elastomeric matrix containing sealed air-filled cavities as well as random labyrinths of small water-filled passages running throughout and in open communication with a surface facing the water. Acoustic waves incident upon the water-facing surface cause time varying shear and bulk deformations within the matrix. As a result of these deformations, acoustic energy is dissipated by hysteretic damping of the elastomeric matrix as well as by viscosity due to water movement to and fro within the passages and into and out of the matrix.
In Cushman (U.S. Pat. No. 5,745,434), an acoustic or damping material is disclosed. The material is produced by mixing at least two species of particles into the material in order to produce the material with tortuous passageways. The particle species are of crumb tire rubber from used tires.
The problem with presently used noise control technologies is that their acoustic properties deteriorate due to the large deformation of the rubber particles or other acoustic impedance particles under the depth and shock pressures associated with undersea operations. As such a composite material as a noise control technology may be acceptable for sound absorption at one hydrostatic pressure or temperature and less effective at another. Additionally, presently used composites may collapse under shock pressure due to the large shear deformation of the rubber particles.